1. Field of the Invention
The invention relates to a twin wire former and method of manufacturing a fibrous material web with a twin wire former, and more particularly, to a twin wire former and method for manufacturing a fibrous material web, such as a paper web, from a fibrous suspension.
2. Discussion of Background Information
Twin wire formers are known in related art apparatus. One such apparatus is disclosed in European Patent No. EP 0489094. This document disclosed a twin wire former with two wire belts, which jointly constitute a twin wire zone. At the beginning of the twin wire zone, the two wire belts jointly form a wedge-shaped inlet gap, which receives the fibrous suspension directly from a headbox. The two wire belt are thus known as a "gap former." Directly at the wedge-shaped inlet gap, the two wires travel by way of a rotatable forming roller. Downstream from the forming roller with respect to the rotational direction thereof, the two wire belts contact forming strips and/or forming shoes inside the twin wire zone.
In other embodiments of twin wire formers, the two wires travel in a generally horizontal direction or ascend diagonally inside the twin wire zone. Consequently, the drainage devices disposed inside the perimeter of the upper wire require relatively high suction to remove the water. In other embodiments, the two wire belts travel in a generally vertical direction upward from the bottom, at the inlet gap. The belts travel by way of a forming roller, which is embodied as a suction roller. In drainage boxes disposed above the forming roller, a few deflection surfaces are necessary to remove the water, possibly requiring a suction connection.
Twin wire formers according to above-noted European Patent No. EP 0489094 may be useful, especially when a forming roller is disposed directly at the inlet gap. Because of the presence of this forming roller, the two wires are guided along a sharply curved path about the circumference of the forming roller immediately downstream of the inlet gap. This sharply curved path is required because the sleeve of the forming roller has a small radius in comparison with the large radius of a forming shoe disposed at the inlet gap. When the suspension stream coming out of the headbox enters the wedge-shaped inlet gap, a large portion of the suspension flows almost in a straight line through the top wire. From the top wire, the water stream breaks up rapidly and uniformly over the width of the machine. Rapid and uniform breakup of the water stream contributes to rapid and uniform web formation. High quality finished fibrous material webs may be produced by this twin wire former, even though the web is rapidly formed.
The disposition of the forming roll at the beginning of the twin wire zone enables both the top wire and bottom wire to be stretched tautly, which creates longitudinal tension, thereby preventing the forming of wire tubes in the travel direction. Additionally, keeping the top wire taut in the lateral direction contributes to uniform web formation, particularly in the critical beginning region of the web formation. Keeping the top wire taut in the lateral direction additionally helps prevent the defect known as "longitudinal striping" in the paper.
However, a disadvantage of the twin wire former known from the above reference EP 0489094 is that during operation, a large amount of energy is used for the production of suction. Furthermore, the starting up of this twin wire former is sometimes difficult when there is a backup of water in the machine.
The trade journal "Das Paper" [Paper] 1970, No. 10A, pp. 779 to 784, disclosed a twin wire former with a twin wire zone that steeply travels in a downward direction. The convexly curved forming shoe that both wires wind around is provided before a forming roller around which both wires wind.